The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer). A pinning layer in a bottom spin valve is typically made of platinum manganese (PtMn). The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Sensors can also be categorized as current in plane (CIP) sensors or as current perpendicular to plane (CPP) sensors. In a CIP sensor, current flows from one side of the sensor to the other side parallel to the planes of the materials making up the sensor. Conversely, in a CPP sensor the sense current flows from the top of the sensor to the bottom of the sensor perpendicular to the plane of the layers of material making up the sensor.
In order to increase data density and data rate even further, in recent years researchers have focused on the use of tunnel junction (TMR) sensors or tunnel valve. A TMR sensor employs an electrically insulating spacer layer rather than a conductive spacer layer. A tunnel valve operates based on quantum mechanical tunneling of electrons through the insulating spacer layer. This tunneling is maximized when the magnetizations of the free and pinned layers are parallel to one another adjacent to the spacer layer.
The ever increasing demand for data storage density and data rate have increasingly pushed the limits of data storage designs. Recently in efforts to overcome such limits, engineers and scientists have focused on the use of perpendicular recording. In a perpendicular recording system a write pole emits a highly concentrated magnetic flield that is directed perpendicular to the surface of the medium (eg. the disk). This field in turn magnetizes a localized portion of the disk in a direction perpendicular to the surface of the disk, thereby creating a bit of data. The resulting flux travels through the disk to a return path having a much larger area than the area in which the bit was recorded.
The increased interest in perpendicular recording has lead to a increased interest in current perpendicular to plane (CPP) magnetoresistive sensor such as CPP GMRs and Tunnel Valves. Such CPP magnetoresistive sensors are particularly suited to use in perpendicular recording. However, the use of such CPP sensors presents certain design challenges. For example, methods used to bias the magnetization of the free layer in a CIP design will not work as well in a CPP sensor design. In a traditional CIP GMR design, biasing is usually provided by a pair of hard bias layers formed at either side of the sensor. The hard bias layers are formed of a high coercivity material (high Hc) and contact the left and right sides of the sensor as viewed from the ABS. Since the current flows through the sensor in a direction parallel with the planes of the sensor, the presence of the hard bias layers (which are electrically conductive) only improves the sense current delivery, and contact of the hard bias layers with the sensor is not a problem.
However, in a CIP sensor design such contact of the laterally opposed hard bias layers with the sensor would cause current shunting, rendering the sensor inoperable. To overcome this, CIP sensor have been designed with thin insulation layers deposited at either side of the sensor, thereby separating the hard bias layers from the sides of the sensor. While this solves the shunting problem, it also leads to insufficient biasing of the free layer.
One attempt to overcome this problem has been to use in stack bias layers. An in stack bias layer is formed, not at the sides of the sensor, but generally above the sensor in the same stack as the sensor. An in stack bias layer generally includes a magnetic layer and a layer of antiferromagnetic material (AFM layer) exchange coupled thereto. A non-magnetic spacer layer separates the in stack bias layer from the free layer to avoid exchange coupling the in stack bias layer with the free layer, which might cause pinning of the free layer.
Such AFM pinned in stack bias layer designs have several drawbacks, however. For example, AFM layers are very thick relative to the other layers in the sensor, and therefore, require a large gap thickness. Since a desired decreased bit size requires decreased gap thickness, any such increase in gap thickness is to be avoided. Another problem with the use of such AFM pinned in stack bias layers if that setting the AFM requires an annealing step that may interfere with the setting of the AFM used to pin the pinned layer. Such AFM pinned bias layers are also sensitive to temperature spikes, such as from a head disk contact, that can cause the AFM to loose it's pinning. In addition, the such AFM layers, when used in an in stack bias layer provide a magnetic biasing of only about 200 Oe. Much stronger biasing would be desired.
Therefore, there is a strong felt need for an in stack bias scheme that can provide strong biasing of a free layer. Such a biasing scheme would preferably avoid many or all of the problems associated with the use of an AFM layer.